Such a lamp base has been disclosed, for example, in WO 97/35336. This document describes a lamp base for a high-pressure discharge lamp having an ignition transformer, which is arranged in the interior of the lamp base and has a closed core. In particular, the ignition transformer is in the form of a toroidal-core transformer. An ignition transformer having a closed core has the disadvantage that, owing to its high inductance during lamp operation after the end of the ignition phase, it impedes the change in polarity of the lamp current if the high-pressure discharge lamp is operated with a current of alternating polarity and the lamp current flows through the secondary winding of the ignition transformer. In addition, with such an ignition transformer the saturation state is reached quickly, with the result that it has a comparatively low energy storage capacity and, after the end of the ignition phase of the high-pressure discharge lamp, a comparatively high current flow occurs which can overload the electrical components of the operating device of the lamp since the inductor effect of the secondary winding of such an ignition transformer is comparatively low. In addition, the application of the transformer windings on to a toroidal core is complex.
WO 02/51214 has disclosed a lamp base having an ignition transformer, which is arranged in the interior of the lamp base and is in the form of a rod-core transformer. This ignition transformer generates a strong magnetic leakage field, which interacts with metallic parts of the lamp base and of the high-pressure discharge lamp and influences the lamp current. In particular, the leakage field causes a current flow in a metallic shielding housing, which surrounds the lamp base for the purpose of improving the electromagnetic compatibility. The current flow in the metallic shielding housing influences the change in polarity, i.e. the current zero phases, of the lamp current and can lead to the high-pressure discharge lamp being extinguished. In addition, the available ignition voltage is reduced owing to the losses in the shielding housing as a result of the magnetic alternating field emanating from the ignition transformer during the generation of the ignition voltage pulses. When a rod-core transformer is used as the ignition transformer, the ignition voltage pulses are considerably damped by the metallic shielding housing.